The invention relates to the precision measuring and monitoring of pressures in the living body, such as intracranial pressure in the head, by means of a long-term, totally implanted sensor which undergoes a conformational change with pressure and which is coupled through the skin by electromagnetic, acoustic, or mechanical transmission to an external device which detects that change and interprets the pressure. The invention refers additionally to a device which is automatically barometric compensated, has immediate zero point reference check, can be made passive, and is insensitive to barometric or temperature changes.
At the present time there is no such wireless device available for general clinical or research purposes. The uses for such a device in neurosurgery would be immediate in the management of intracranial hypertension, monitoring of intracranial pressure in all cases of intracranial neurosurgery and head trauma, long-term diagnostics for evidence of tumor recurrence, and management of hydrocephalus.
All devices previously proposed have significant short-comings which make them impractical for widespread, safe, accurate, reliable, and long-term use as intracranial pressure monitors. Most designs involve a tube or wire connection through the skin to an external device, and since this greatly increases the chance of infection and electrical shock to the patient and reduces the patient's mobility they are hazardous and impractical. Of the devices which are wireless and fully implanted, they usually involve a sealed inner volume containing a fixed amount of gas, this being housed in a flexible container which deflects under pressure. The major problems with this design aspect are the following: liquids and gases will inevitably diffuse through the membranes and walls of the container causing steady drift of the zero-point reading, and causing an unpredictable error in the device's calibration; changes in barometric pressure will cause significant variations in the body pressure relative to the fixed volume pressure and thus the device's pressure readout must be corrected for barometric pressure changes in the external detection system; a trapped volume of significant size could make it dangerous for a patient to experience atmospheric pressure change, such as those found in air travel, for fear of rupturing the device; and temperature changes in the patient will cause changes in the trapped volume and resultant errors in the pressure reading. Previous totally implanted designs provide no means to check out their zero-pressure calibration after implantation and thus no means to determine diffusion or temperature drifts in the readings nor any check of the proper function of the device, which is essential for long and short-term implantation. Most previous designs are of complex construction, involve high tolerance parts and assembly, and are not amenable to calibration standardization; all of which make them expensive, inaccurate, and unsuitable for simple and general application.
Accordingly, some of the principal objects of the present invention are the following:
(1) To provide a pressure detector which can be implanted for an indefinite period under a fully intact skin with no wire or tube connections to the exterior so as to reduce infection and electrical shock hazard, and to read pressures in inaccessible spaces on the body, such as intracranial pressure, with an accuracy of 5 to 10% or better.
(2) To eliminate or make insignificant all inaccuracies, and dependencies on a trapped volume of gas or fluid in the device, to make the pressure readings insensitive to drifts from membrane permeability, barometric change, and temperature variation, and to eliminate the hazard of rupturing the device during air travel.
(3) To provide automatic barometric compensation as a built-in feature of the implanted device.
(4) To provide a means of easily and instantly checking the zero-pressure calibration of the device.
(5) To provide a sufficiently fast dynamic response to enable observation of variations in the body pressure due to heart rate, respiration, and any other physiological changes.
(5) To allow a simple calibration standardization of the implant.
(7) To allow the implanted device to be of simple, passive, compact, and low cost construction so as to be implanted permanently and to function properly for indefinitely long periods.
(8) To make the system amenable to telemetry over long distances so as to monitor pressures in a freely moving patient.